Pavement markers for the delineation of traffic lanes on automobile highways or airport landing strips and traffic areas in parking zones are a common article of manufacture in modern life. The requirements for this common item are, however, quite demanding.
A pavement marker should be highly visible to an oncoming motorist or pilot during the daytime when ambient light strikes the marker from all directions. Most of the light at this time is coming from overhead and the source is, of course, the sun. On the other hand, the marker must retain its visibility at night when the dominant direction of the light coming to the marker is from the direction of the observer and the source is the headlights of the observer's vehicle.
In order to be visible in the first situation the marker must be highly reflective and scatter the overhead light parallel to the ground surface since that is usually where the observer will be located. In the latter case the marker must be retroreflective which, as might be surmised from the Latin word retro meaning backward, refers to a device that returns light to its source. It follows from this concept that a totally retroreflective marker is a poor performer during the day because the reflected light would be directed towards the sun rather than towards an observer. Similarly, a purely reflective marker is a poor performer at night since it tends to scatter the light from a single source.
In addition to being highly visible, a pavement marker must also warn the vehicle operator of its presence by the senses of touch and hearing. This requirement comes into play when the vehicle wheels inadvertently, or otherwise, contact the marker. This situation must not result in damage to the vehicle; instead, the marker physical design must cause a combination of vibration and sound that alerts the operator of the vehicle to his location relative to a traffic lane, pedestrian safety zone or the like. Not only must this situation not cause vehicle damage but the impact must not be so abrupt as to unduly startle the operator or transfer a force to the steering wheel that would result in loss of control.
Markers are usually mounted to the surface of the road or runway with an adhesive. The design of the marker must provide a relatively broad base that has vertical and horizontal surfaces to which the adhesive can cling so that the marker does not move after the adhesive cures.
In addition the design must anticipate that markers are installed by the hundreds and thousands and the installer does not have the time to take unusual care with this process. Therefore, the marker may be pressed hurriedly and forcibly into the adhesive with the result that adhesive may well upward along the marker sides. This may obscure some of the reflective surface. More importantly, since the retroreflective surface is often close to the mounting surface of the marker in order to gather and reflect light from approaching headlights, a large part of this surface may be obscured. The marker design, thus, should tend to deflect the upward welling adhesive away from the retroreflective surfaces of the marker.
Most retroreflective materials are less durable than reflective surfaces such as glazed ceramic and require some protection from the impact and abrasion of vehicle tires. A complete marker provides such protection.
The marker must be durable. It must live in the hostile environment of rain, snow, hot and cold cycles, and the impact of vehicle tires and survive; not only survive but keep its reflective qualities despite the abrasive contact of tires. The design must be such as to inhibit the buildup of dirt and debris that would obscure the marker and, instead, facilitate the cleaning away of these materials by the impact and wiping motion of vehicle tires.
Finally, because of the great number of markers required for delineation of highway lanes and other applications, the marker design must be economical to produce.
Since at least some of these parameters place conflicting demands upon the marker design, all inventions are compromises to some extent. One marker invention is U.S. Pat. No. 3,332,327 to Heenan. This invention utilized corner reflectors molded into the inner wall of a hollow structure which was then filled with an epoxy compound to provide strength. Corner reflectors are open devices composed of three mutually perpendicular walls. If the walls are perfectly flat and precisely perpendicular to each other, any entering radiation, whether in the microwave portion of the spectrum or the visible light portion, will be returned in the direction of the source. This makes a useful structure for range testing of radar systems or returning incident light from a lane marker. The light reflecting properties of corner reflectors are discussed in U.S. Pat. No. 1,906,655 to Stimson.
U.S. Pat. No. 3,938,903 to Montgomery describes the use of circular studs on the pavement contacting base of a marker to provide an adequate surface for the adhesive to successfully hold the marker to the pavement.
Montigny in U.S. Pat. No. 4,008,973 has a low profile marker with protected retroreflective structures. Heenan in U.S. Pat. No. 4,208,090 proposes a compromise between corner reflectors and flat surfaces which are superior for daytime reflection purposes. U.S. Pat. No. 4,653,955 to Racs calls for the use of a retroreflective strip bonded to the sides of a circular marker. Heasley in U.S. Pat. No. 3,980,393 combines the corner reflector concept with a convex and concave lens structure which protects the corner reflector structure. U.S. Pat. Nos. 4,232,979 and 4,340,319 to Johnson and Heenan involved the use of abrasion resistant untempered glass sheet for protection of such corner reflectors. Forrer in U.S. Pat. Nos. 4,753,548 and 4,797,024 describes the use of a polymerizable acrylic protective covering for a retroreflective lens. Design patents U.S. Pat. No. Des. 215,376 and U.S. Pat. No. Des. 225087 to Haley illustrate two possible design outlines for lane markers.
Some of these inventions are complex and rather expensive to produce or are specialized for nightime use only. Others are larger than necessary to provide visual and vibration feedback to the vehicle operator. Others have no provisions for preventing flow of adhesive over the retroreflective surface.
Thus, a pavement marker that combines good daytime and nightime reflectivity, provides abrasion protection for the retroreflective material, has a low smooth profile so as to not induce excessive shock and vibration to vehicle wheels, can be set into pavement adhesive without having its surfaces obscured and is simple and economical to produce would be of considerable utility.